Why do I get an "Unprotected Formula" message in Excel 2016?

Symptoms

You see a green triangle in the upper-left corner of a cell that contains a formula.

You are evaluating errors in the Error Checking box.

Cause

By default, all cells are locked to protect them from accidental or unauthorized changes. In this case, the cell containing the formula is not locked for protection.

Resolution

Lock the cell by doing one of the following:

Click the Error Checking button
next to the cell, and then click Lock Cell.

In the Error Checking box, click Lock Cell.

Protecting cells that contain formulas prevents them from being changed, and can help avoid future errors. However, locking the cell is a first step, and you must perform additional operations to protect the workbook, such as setting a password.